Question: Which of these numbers is composite? ${37,\ 47,\ 71,\ 73,\ 81}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 37, 47, 71, and 73 each have only two factors. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 81 is the composite number.